Processes and apparatus for the in situ generation of agglomerate structures and the controlled influencing of the resulting agglomerates are particularly useful in catalytic gas-solids syntheses where difficulties arise in the case of conventional particle precipitation techniques, since the size of the primary particles of the aerosols to be treated is, as a rule, distinctly less than 1 .mu.m, and it is not possible, or at least not economically viable, for particles of this size to be precipitated applying conventional particle precipitation techniques. In this field of application, i.e. the solids synthesis from gas phase reactions, the size of the primary particles is frequently only a few nanometres. Already for economic reasons, an extremely effective particle precipitation is required in such instances, since the particles present in the aerosol constitute the substance of value to be extracted. Accordingly, within the framework of such solids syntheses, the increasing of the particle size prior to the actual particle precipitation is the primary objective, although the resultant structure of the agglomerate may be influenced by the agglomeration process selected.
Basically, the desired increasing of the particle size can be achieved in various ways. In addition to so-called "wet" processes, in which the particle-size increase is achieved by condensation of water vapour from a supersaturated atmosphere, agglomeration techniques, which are referred to as "dry" processes and in which the desired agglomeration is provided by a collision of the particles in a fluid phase, are also known. Accordingly, a prerequisite for this so-called direct agglomeration is that the individual particles in the fluid phase have a relative velocity amongst one another. This relative velocity can be provided by means of thermal and turbulent diffusion or by a particle movement induced by force fields. The force fields include, in particular, gravitational fields, centrifugal fields, sound fields or electric fields. The advantage of an electrically induced agglomeration, i.e. the production of relative velocities of the particles by means of an electric field, lies in the energy requirement which is considerably lower, for example in comparison to sound fields, in particular in the range of relatively small and smallest particles, where electric forces continue to exert a considerable influence on particle movements, while having a low energy requirement.
Electrostatic filters, in which a particle-loaded flow of gas, which is to be cleaned, is divided up into two component gas flows, which are spatially separated from each other, are known from DE 37 37 343 Al and U.S. Pat. No. 3,826,063. Each component gas flow is charged unipolarly in the usual manner by at least one pair of electrodes. After the unipolar charging of the component gas flows, they are again brought together.
An electrostatic filter which comprises ionization electrodes and precipitation electrodes is known from DE OS 1 407 534 for the precipitation of particles from flows of gas. The ionization electrodes are designed as needle-shaped electrodes which are arranged opposite each other, while in each case two oppositely disposed ionization electrodes project into a hollow body which serves as the precipitation electrode. The desired precipitation of the particles is achieved due to a potential difference between the ionization electrodes and the precipitation electrode associated therewith, i.e. also in this arrangement, a unipolar charging of the particles takes place.
A process and an apparatus for the separation of solid or liquid particles from a flow of gas by means of an electric field are known from U.S. Pat. No. 4,734,105. In this regard, the particle-loaded gas flow is directed through a flow duct in which a plurality of planar flat or planar curved electrode pairs are arranged. At least the main electrodes are provided with needle-shaped extensions which project into the flow duct and have spherical or semi-spherical tips, at which a corona discharge and, thus, an ionization of gas molecules take place, once an electric field has been applied. In this regard, the spherical or semi-spherical tips of the needle-shaped electrode extensions have a diameter which is greater than the diameter of the shank of the needle. It is intended, by means of two secondary electrodes which are designed, for example, to be lattice-shaped, to ensure that the region in which the gas is ionized is separated in the radial direction from that region of the flow duct in which the particles, which are charged with the aid of the gas ions, collide. It is intended, in this manner, to permit the application of an intense electric field, which is intended to provide the solution to the problem set out in U.S. Pat. No. 4,734,105, namely to reduce, to a substantial extent, the path required in the direction of flow for the precipitation of the particles. Accordingly, the apparatus described in U.S. Pat. No. 4,734,105 is a further development of an electrostatic filter.
Accordingly, the basic concept of increasing the rate of collision of particles which are present in a flow of gas by applying an electric field, is known. It has, however, heretofore not been possible, with the aid of known methods of charging particles, to influence the electrically induced agglomeration of particles in a controlled manner such that agglomerates having specific properties with regard to concentration, structure or dimension are obtained.
It is thus an object of the present invention to provide a process and an apparatus for the in situ generation of agglomerate structures and the controlled influencing of the resulting agglomerates.